The present invention relates to coating machines for paper or the like, and more particularly to improvements in apparatus for removing surplus coating compound from the coated side or surface of a web or sheet of paper, cardboard or the like. Still more particularly, the invention relates to improvements in the construction, mounting and adjustment of doctor blades which are used in such apparatus to remove the surplus of coating compound from a running web or sheet of coated material.
Many types of paper or cardboard must be provided with relatively thick or heavy layers of a coating compound. For example, it is not out of the ordinary if the weight of the coat on a web of paper or cardboard is in the range of between 15 and 25 grams per square meter. The application of such heavy coats presents many problems; as a rule, the operation involves repeated coating of paper or cardboard with suitable compound, e.g., a mixture of white mineral matter, adhesive and dyes. When the weight of the coat exceeds a threshold value, normally about 15 grams per square meter, the coating machine must embody an apparatus which effects a correction of profile across the full width of the running paper or cardboard web. This insures that the coated material exhibits a smooth surface and can be properly wound to form satisfactory rolls.
German Pat. No. 2,012,598 discloses an apparatus for removal of surplus coating compound from coated paper wherein the surplus removing means comprises a doctor blade extending across the full width of the running paper web and a gas-filled hose which biases the blade against the coated surface. The force with which the blade bears against the running web can be regulated by changing the pressure of compressed gas (normally air) in the interior of the hose. The hose is mounted on a strip-shaped support and bears directly against the doctor blade. The support consists of several sections which are movable relative to each other. This enables the attendants to control the force with which the corresponding portions of the blade bear against the coated surface and to thus determine the exact profile of the coated surface.
The patented apparatus cannot insure the formation of a high-quality coat because the wall thickness of a relatively long hose is not uniform from the one to the other end thereof. Thus, when the pressure of fluid in the interior of the hose increases, the expansion of thin-walled portions exceeds the expansion of thicker portions with the result that the profile of the coated surface deviates from an optimum profile. In order to compensate for non-uniformity of wall thickness of the hose, the patent proposes the utilization of a springy insert which is made of sheet metal and is installed intermediate the blade and the hose. The insert is capable of compensating for some deviations of wall thickness of the hose from the optimum thickness; however, the compensation is insufficient to insure that the profile of the coat surface will not deviate appreciably from a desired optimum profile. Furthermore, internal stresses which develop in the material of the springy insert reduce its effectiveness.
The undesirable effect of variations in wall thickness of the hose is more pronounced when the pressure in the interior of the hose rises. Therefore, the aforementioned strip-shaped support must be assembled of a large number of relatively short sections in order to enable the attendants to compensate for departure of certain portions of the doctor blade from an optimum position. This contributes to complexity of the patented surplus removing apparatus. Also, the versatility of the patented apparatus is unsatisfactory because it cannot be readily converted from operation with a slightly bent doctor blade (the edge of the blade bears against the coated surface) to operation with a pronounced bending of the blade (whereby a side of the blade bears against the coated surface) or vice versa.